Johannesburg, South Africa to JFK with a stop in Dakar, Senegal. Left at about 9pm, arrived at 7:30. This was in winter, with sunlight during the final hour. Total time: about 19 hours including the stop.

My longest were two flights with NZ. LAX-AKL and LAX-SYD. Both are leaving at around 10 PM and arrive in the early morning. As I did both flights during the southern hemisphere winter, it was dark all flight.